Several kinds of electron beam exposure systems are known in the art. Most of these systems are provided to transfer very precise patterns onto an exposure surface of a substrate. Since lithography features are pushed to become smaller and smaller following Moore's law, the high resolution of electron beams could be used to continue the drive to even smaller features than today.
A conventional electron beam exposure apparatus has a throughput of about 1/100 wafer/hr. However, for lithography purposes a commercially acceptable throughput of at least a few wafers/hr is necessary. Several ideas to increase the throughput of an electron beam exposure apparatus have been proposed.
U.S. Pat. No. A1-5,760,410 and U.S. Pat. No. A1-6,313,476, for instance, disclose a lithography system using an electron beam having a cross section, which is modified during the transferring of a pattern to an exposure surface of a target. The specific cross section or shape of the beam is established during operation by moving the emitted beam inside an aperture by using electrostatic deflection. The selected aperture partially blanks and thereby shapes the electron beam. The target exposure surface moves under the beam to refresh the surface. In this way a pattern is written. The throughput of this system is still limited.
In US A1-20010028042, US-A1-20010028043 and US-A1-20010028044 an electron beam lithography system is disclosed using a plurality of electron beams by using a plurality of continuous wave (CW) emitters to generate a plurality of electron beamlets. Each beamlet is then individually shaped and blanked to create a pattern on the underlying substrate. As all these emitters have slightly different emission characteristics, homogeneity of the beamlets is a problem. This was corrected by levelling every individual beam current to a reference current. Correction values for the mismatch are extremely difficult to calculate and it takes a significant amount of time, which reduces the throughput of the system.
In Journal of Vacuum Science and Technology B18 (6) pages 3061-3066, a system is disclosed which uses one LaB6-source for generating one electron beam, which is subsequently, expands, collimated and split into a plurality of beamlets. The target exposure surface is mechanically moved relatively to the plurality of beamlets in a first direction, the beamlets are switched on and off using blanking electrostatic deflectors and at the same time scanning deflectors sweep the beamlets which have passed the blanker array over the target exposure surface in a direction perpendicular to the first direction, thus each time creating an image. In this known system, electrostatic and/or magnetic lenses are used to reduce the image before it is projected on the target exposure surface. In the demagnification process at least one complete intermediate image is created, smaller than the one before. When the entire image has the desired dimensions, it is projected on the target exposure surface. A major disadvantage of this approach is that the plurality of electron beamlets together has to pass through at least one complete crossover. In this crossover, Coulomb interactions between electron in different beamlets will disturb the image, thus reducing the resolution. Moreover, due to the strong demagnification of the image, the area that is exposed at one time is rather small, so a lot of wafer scans are needed to expose a die: 16 scans are needed to expose one die, requiring a very high stage speed for reaching a commercially acceptable throughput.
In GB-A1-2.340.991, a multibeam particle lithography system is disclosed having an illumination system, which produces a plurality of ion sub-beams. The illumination systems use either a single ion source with aperture plates for splitting a beam in sub-beams, or a plurality of sources. In the system using a single ion source, the aperture plate is projected (demagnified) on a substrate using a multibeam optical system. The system furthermore uses a deflection unit of electrostatic multipole systems, positioned after the multibeam optical system, for correcting individual imaging aberrations of a sub-beam and positioning the sub-beam during writing. The publication does not disclose how each sub-beam is modulated. Furthermore, controlling individual sub-beams is a problem, and maintaining inter-sub-beam uniformity.
In Jpn. J. Appl. Phys. Vol. 34 (1995) 6689-6695, a multi-electron beam (‘probes’) lithography system is disclosed having a specific ZrO/W-TFE thermal emission source with an emitter tip immersed in a magnetic field. A disadvantage of such a source is its limited output. Furthermore, this source needs a crossover. The mutual homogeneity of the ‘probes’ is not further discussed. Furthermore, the intensity of the source is a problem.
The article furthermore in a general way mentions a writing strategy in which a stage is moved in one direction, and deflectors move the ‘probes’ concurrently through the same distance perpendicular to the direction of the stage movement. A further problem, not recognised in this publication, is correction of deviation of electron beamlets from their intended positions.